Sealing devices utilizing ferrofluid are known as described, for example,, in Japanese Patent Publication No. 24585/1986. Such a sealing device has an annular sealing member 52 disposed so as to surround the circumference of a shaft 51, as shown in FIG. 6. This sealing member 52 consists of an annular magnet 53 and a pair of pole pieces 54,54 bonded onto both sides of the annular magnet 53, with a ferrofluid 55 being interposed between the internal circumferential surfaces of the annular pole pieces 54,54 and the external circumferential surface of the shaft 51, whereby to function as a sealer.
In the constitution of the conventional sealing member 52, an annular magnet 53 and annular pole pieces 54,54 are shaped in separate steps, and then they are bonded together, owing to the reason described below. Namely, since the inner diametrical size of the sealing member 52 is a factor which defines the size of the gap between the sealing member 52 and the shaft 51 and significantly influences the sealing performance of the sealing device, whereas the outer diametrical size of the sealing member is a factor which significantly influences the assembling feasibility with the member for fixing the sealing member 52, it is necessary to maintain the inner and outer diametrical sizes of the sealing member 52 to have good accuracy. However, in the conventional sealing member 52, the annular pole pieces 54,54 are formed to have optimum dimensional accuracy, whereas the annular magnet 53 is formed to have an inner diametrical size slightly greater than that of the annular pole pieces 54, 54 and an outer diametrical size slightly smaller than that of the annular pole pieces 54,54, whereby the internal and external circumferential surfaces of the annular magnet 53 are adapted not to extend over the internal and external circumferential surfaces of the respective annular pole pieces 54,54, correspondingly, to maintain dimensional accuracy.
However, even in such sealing members as obtained according to the production process described above, sufficient accuracy may sometimes not be attained if the circumferential surface of the annular magnet 53 happens to extend over the circumferential surfaces of the annular pole pieces 54, 54 during the bonding operation. The above process also involves a problem that it requires much difficulty in the bonding operation.
This invention has been accomplished for the purpose of solving the above problems of prior art and is directed to provide a sealing member for a ferrofluid seal which can be produced with easier procedures and has improved dimensional accuracy and a method of producing the same.
These and other objects of this invention will become more apparent from the following description of the preferred embodiment.